The dopaminergic system is a key regulator of distinct neural processes such as learning and memory, reward and addiction, locomotion, and prolactin secretion. Consequently, a dysregulation among the four dopaminergic pathways contributes to the pathophysiology of irrevocable neurological disorders and psychiatric illnesses such as Schizophrenia, Parkinson's disease, and drug addiction. Specifically, D2 dopamine receptor is known to be a primary target of antipsychotic drugs; however, conventional therapeutics still exhibit limited efficacy and typically cause adverse side-effects. Most preclinical and clinical studies consider D2 receptor as a single entity, but alternative splicing of the D2 gene generates two isoforms-a long form (D2L) and a short form (D2S). Knowing the relevance of D2 receptor's signaling in the regulation of physiological responses to dopamine, and the current use of D2 receptors agonists and antagonists in therapy, it is essential to explore whether D2L and D2S can be considered as equally functioning in vivo. Recent studies from our lab have indicated a distinct functional difference between D2S and D2L isoforms, where D2S and D2L have a presynaptic and postsynaptic role, respectively. However, without isoform-selective ligands, it is difficult to accurately discriminate the in vivo function of the two isoforms. Correspondingly, our lab has generated D2 isoform specific knockouts to compare physiological responses and biochemical differences to determine which receptor isoform is acting pre-/post-synaptically, how their activation is distinct in terms of sigal transduction and in response to drugs of abuse, and finally, which receptor isoform is targeted by specific antipsychotics. The following specific aims are proposed to characterize the D2 isoforms: In aim 1, we will analyze the role of each D2 isoform in the nigrostriatal pathway in vivo, by comparing the locomotion of D2S-/- and D2L-/- with its respective wild-type (WT) in response to dopaminergic agonists and antagonists. We will also analyze D2L- and D2S-specific signaling under basal conditions and in response to pharmacological challenges. In aim 2, we will analyze the role of each isoform in the mesolimbic pathway, where we will examine the motor, rewarding and sensitization responses to cocaine and dissect cell-specific induction of immediate-early-genes and transduction pathways induced by cocaine. Successful completion of these studies will give important information on the D2 receptor-mediated control of the dopaminergic system. Thus expanding our scientific knowledge of dopamine-mediated responses and providing crucial information for pharmacological designing of D2-targeted drugs.